Thermoplastic foams are finding increasing use in the manufacture of a wide range of articles. Typically, foam products are formed by adding a "blowing agent" to a thermoplastic resin and subjecting the resultant mixture to conditions under which the resin is expanded. For example, in an extrusion foaming process, a blowing agent is mixed with a molten thermoplastic resin under pressure and the mixture is then cooled and extruded through a forming die into a zone of reduced pressure. The blowing agent expands in the zone of reduced pressure, thereby expanding the thermoplastic resin to produce a cellular thermoplastic structure having far less density than the resin from which the foam is made. The foam structure is maintained by replacing with air the blowing agent in the cells of the foam.
Thermoplastic foam products can be used in a variety of applications, such as insulating materials, packaging and cushioning materials, and the like. For example, polyolefin foam products provide good insulating and cushioning properties desirable for these applications. In many packaging applications, such as packaging of electronic devices, it can be advantageous to use foam packaging that exhibits antistatic characteristics. However, polyolefin foam products readily acquire static charges, for example during processing of the foam product. This can result in a variety of problems, including poor handling properties, adherence of particles of dust and other foreign matter to the polymer, interference with electronic devices packaged therein, and the like.
Various techniques have been developed to address the tendency of thermoplastic polymers to accumulate static charges. One technique is the development of more electrically conductive polymers. Another technique is copolymerization of an antistatic resin with the base polymer. These techniques, however, can be limited by multiple, complex and expensive reagents and processing conditions.
Other techniques have addressed this problem by incorporating a small quantity of an antistatic agent into the thermoplastic resin. The antistatic agent migrates to the surface of the plastic product to modify its electrical properties. Conventional antistatic agents used in processing polyolefin resins include fatty ammonium compounds, fatty amide or amines, and phosphate esters. However, while antistatic properties can be provided using these antistatic compounds, the resultant polyolefin foam products do not exhibit good stress-cracking performance properties. For example, many electrical devices include a polycarbonate substrate. Amine and amide-type antistatic agents are not compatible with polycarbonate, and when contacted to the surface of a polycarbonate device, the polycarbonate product swells, thereby causing cracking in the packaging.
In addition, amine and amide-type antistatic agents can have a corrosive impact on metal substrates. Thus in many packaging applications, the item being packaged may not be able to tolerate direct contact with the foam itself.
Accordingly, despite these and other techniques for forming antistatic polyolefin foam products, it would be desirable to provide an antistatic polyolefin foam product which not only exhibits good antistatic properties but also good stress crack resistance. Further, it would be desirable to provide an antistatic polyolefin foam product which does not have a corrosive effect upon metal substrates. In addition, it would be advantageous to provide foam products exhibiting these and other desired characteristics which could be readily prepared using existing processes and equipment.